The present invention relates to a bonding wire, a sub-assembly comprising the bonding wire, and a method for producing the bonding wire.
Bonding wires are often used in microelectronics for electrical contacting of semiconductor chips to a support substrate, such as, for example, a wiring support.
It is customary to use round thin wires or thick wires differing in cross-section. The diameter of thin wires is in the range of 17-100 μm, whereas thick wires are characterized by diameters of 100-500 μm.
In recent years, it has become established that it may be of advantage, depending on the specific application, to use round bonding wires that may consist of multiple sheets or layers of different materials.
Accordingly, it is known, for example from DE 42 32 745, that the reliability of ultrasound bonding may be improved through the use of bonding wires made of gold or copper having a jacket made of aluminum for this purpose.
EP 1 279 491 proposes, for cost reasons, to reduce the amount of gold in bonding wires by providing gold-coated bonding wires made of copper, silver or palladium rather than pure gold wires.
According to JP 62-97360, bonding wires made of copper are protected from oxidation by coating them with a layer made of an oxidation-resistant material.
Although bonding wires having a round cross-section, as described above, are suitable for many applications, the use of so-called ribbon wires has also been established for many years. These wires are characterized in that they do not have a round cross-section, but rather have an elliptical or rectangular cross-section, and are used mainly for bonding in power components. The larger, but also flatter, geometry of ribbon wires has a practical advantage in that the processing is more rapid since only a small number of ribbon wires, rather than many thinner bonding wires, need to be bonded to attain contacting. Moreover, the surfaces to be contacted through the bonding wire (bond pads) are better covered by ribbon wires since there is hardly any widening at the base. Further, the flat geometry of ribbon wires allows lower loops to be generated. In addition, the heel damage at comparable loop geometry, i.e., the damage in the sensitive arc of the bonding material, is clearly less pronounced than with comparable thick wires. This allows unusually high steps to be bridged by short bonds and thus to implement densely packed geometries in housing bonding.
In order to combine the above-mentioned advantages associated with the use of bonding wires having multiple layers made of different materials and the advantages associated with the use of ribbon wires, it has been proposed to produce ribbon wires having a sandwich structure.
DE 10 2006 060 899 A1, for example, discloses a bonding wire that comprises a solderable coating on one side and a bondable coating on the opposite side.
DE 10 2006 025 870 A1 also describes a bonding wire with a sandwich structure and comprises at least three layers, namely two outer layers and at least one intervening layer. This bonding wire may be produced by hot rolling, which ensures that the different layers of the bonding wire are exclusively arranged to be parallel to each other, and thus are visible in a side view of the wire.
However, tests have shown that prior art bonding wires having a sandwich structure are associated with various disadvantages, in addition to the positive properties that have been achieved thus far. For example, it has been noted that the bonding of bonding wires having a sandwich structure is often associated with damage to the surfaces of the chips and/or supports (bonding surfaces) to be connected. Moreover, it has been evident that the risk of short-circuiting is markedly higher when sub-assemblies containing ribbon wires having a sandwich structure are used.